A high efficiency driver in a relatively small horn (for the wavelength) means flat 38Hz FR is a stretch. It is probably an optimistic F10 spec. The F6 (the -6dB rolloff freq) that you would use for the sub crossover freq is probably more like 45Hz. Crossovers sum flat if they cross (same amplitude) at F6, aka the -6dB frequency.
A horn's natural rolloff is -24dB per octave. That is the slope, it means it gets 24 decibels quieter when the frequency drops by one octave (cut in half.) It is important for the slope of the sub's low pass filter and the main speakers natural high pass rolloff to match in slope, so that you end up with a flat amplitude summation of both speakers thru the crossover band. This means set the sub slope filter to 24dB. I would try the sub turned down to lowest freq, see if the FR is flat from your listening position. Room modes, mfg extension exaggeration, and tolerance of the sub filter circuit could make it work without additional filters. Don't worry about a little boost around 40-60Hz, you'll like it.
If it has flattish FR but still doesn't sound "clear" or if it sounds muddy or blurred bass, then you need to adjust phase so the sub and mains play perfectly in sync and the sound arrives to ears at the exact same time. The easiest way to ensure that is to put the sub at same distance from ears as the mains, and set sub phase to zero. If you need to hide it, then try putting it behind your listening chair and adjusting the phase delay above zero until it sounds clear again. When it's close to you then you can turn it down and reduce room mode problems and lengthen reflection times. It's a great wtg. Below 80Hz you can't tell which direction the sound comes from so behind you works fine.